In the RFID system, radio communication is performed between an RFID tag (hereinafter sometimes simply referred to as a “tag”) and a reader/writer. Particularly, in an RFID system in which a passive or semi-passive RFID tag is used, conveniently the RFID tag can be started up by a radio wave from the reader/writer to perform communication. Therefore, the RFID system is used in various applications.
For example, in a production plant, an article is conveyed by a conveyer while the RFID tag is attached to each article of a management target, an antenna of the reader/writer is installed in a vicinity of the conveyer, and the communication is performed between the RFID tag and the antenna every time the RFID tag enters a communication range of the antenna with movement of the article, thereby executing a process of reading/writing information (see Patent Document 1).
There is also well known a system in which, in order to manage entrance and exist, the antenna of the reader/writer is installed in a passageway to an entrance door, and the RFID tag attached to the mobile object (person or article) that passes through the passageway reacts to a command from the antenna.
For example, Patent Document 2 describes that, in a system that acquires identification information from the RFID tag passing through a plurality of gates located close to each other, a read signal is transmitted from the antenna installed in each gate, pieces of identification information are acquired by response signals from all the RFID tags that receive the read signal, and a filtering process of removing the identification information from the unnecessary RFID tag is executed.
Patent Document 3 describes that an interrogator (reader/writer) and a fixed tag are installed in each of the plurality of gates through which a person passes, and in each interrogator, a mobile tag that is possessed by the person passing through the gate and the fixed tag are simultaneously read a plurality of times per unit time, and whether the mobile tag exists in each gate is determined by availability of communication with each tag or a communication success rate. Patent Document 3 also describes that, when determined that the mobile tag does not exist due to difficulty of communication with the fixed tag, it is recognized that the radio wave to the fixed tag is interrupted by a person who does not possess the mobile tag, and the tag responding to the command from the interrogator is shifted in each type of the tags by allocating individual time slots to the fixed tag and the mobile tag.